Princeton University
COS 217: Introduction to Pgmming Systems

Purpose

Help you learn/review (complex) C variable and function declarations and definitions

C Declarations and Definitions

(See Harbison and Steele book for an even more formal presentation)

C Variable Declarations and Definitions

Declaration vs. definition

Scope, linkage, and duration

Variables with process duration exist in either the data or bss sections

Global variable that is explicitly initialized goes in data section

Global variable that is not explicitly initialized goes in the bss section, and is initialized to 0

Examples:

int i = 5;  /* data section */
int j;      /* bss section */
int k = 0;  /* data section */

"bss" stands for "block started by symbol"; maybe was descriptive at one time, but now is not

Now we've seen all 6 sections: text, rodata, stack, heap, data, bss

Global variables

Review examples of global variable declarations and definitions

Review examples of combinations of global variable declarations and definitions

Style: Minimize use of global variables; when you use them, try to make them static (and const)

Global variables with external linkage are a maintenance nightmare

(Code that uses global variables is not thread-safe)

Happily, Splint complains about such variables

Local variables

Review examples of local variable declarations and definitions

Note the two distinct meanings of the "static" keyword

Applied to a global variable => the variable has internal linkage

Applied to a local variable => the variable has process duration

And both of those meanings differ from the meaning of "static" in Java!!!

C Function Declarations and Definitions

Note especially static function definitions

Complex Declarations and Definitions

The "right-left rule"

Read the three steps that define the rule; do so quickly; the examples are more helpful

Some examples (if time is short, cover only the boldface examples):

int x[5];
First eliminate the subscript:
int x[];
Now do the translation:
   x        x is
   x[]      x is an array of
   int x[]  x is an array of int
Now add back the subscript, and refine the description
   x is an array of 5 ints

int x(int i);
First eliminate the parameter(s):
int x();
Now do the translation:
   x        x is
   x()      x is a function
   int x()  x is a function returning an int
Now add back the parameter(s) and refine the description:
   x is a function accepting an int and returning an int

int *x();
   x         x is
   x()       x is a function
   int *x()  x is a function returning a pointer to int

int *x[];
   x          x is
   x[]        x is an array of
   *x[]       x is an array of pointer to
   int *x[]   x is an array of pointer to int

int (*x)[];
   x            x is
   *x           x is a pointer
   (*x)[]       x is a pointer to an array
   int (*x)[]   x is a pointer to an array of int

int (*x)();
   x            x is
   *x           x is a pointer
   (*x)()       x is a pointer to a function
   int (*x)()   x is a pointer to a function returning an int

int (*x())[];
   x               x is
   x()             x is a function
   *x()            x is a function returning a pointer
   (*x())[]        x is a function returning a pointer to an array
   int (*x())[]    x is a function returning a pointer to an array of int

int (*x())();
   x               x is
   x()             x is a function
   *x()            x is a function returning a pointer
   (*x())()        x is a function returning a pointer to a function
   int (*x())()    x is a function returning a pointer to a function
                   returning an int

Observation 1: you should know how to understand such complex declarations

Observation 2: you should never write such things!!!

In practice, they're easy to avoid

If unavoidable, use typedefs to simplify

Copyright © 2009 by Robert M. Dondero, Jr.